[shutterspeed]

I'm trying to create a gas tank cleaning solution and combined 1 to 1 gasoline and 32.5% muriatic acid in a test jar. The two do not mix, as I expected, the acid is the heavier of the two.  However, left to sit overnight, there is now a middle dark brown layer in between the acid and the gasoline. I'm exceedingly curious what that substance might be.

Does anyone have any ideas?

[Arkcon]

I'm not one of this forums best organic chemists, but I do have some ideas, all dubious and bad news, however.

For one thing, gasoline isn't one thing, but a mixture of a variety of different chemicals.  So a simple answer is not in the cards.  As a general rule, non-polar compounds like the linear alkanes, cyclic alkanes, alkenes, aromatics and additives won't mix well enough with ionic compounds in water.  However, over time some strange chemistry may happen.  If you look up each general class of compounds above, you may find out what sort of reactions are possible.

To take another tack ... cut it out.  Don't mix random chemicals without knowing what you're doing.  If you'd had used concentrated sulfuric, you might have started an instantaneous flash fire.  If you had used concentrated nitric, you might have had an even worse reaction. 

To me, it seems ill-advised, on the face of it, to clean out a gasoline tank with strong mineral acid.  If its made of mild steel or iron, HCl will corrode it, then you'll have rust particles being fed. If the tank were made of aluminum, this would likewise be a disaster.  Just not a good plan all around.